Anyone working in extrusion or injection molding knows that the screw is the heart of the equipment.
Once this heart has problems—such as wear or enlarged clearance—output will drop inexplicably, black specks in the material will increase, and no matter how you adjust the process temperature, you can’t keep it under control. To pull the screw out and measure it, you need to shut down, cool down, clean, and extract it—at least half a day, maybe a full day. All that wasted time is real production capacity down the drain.
So how can we precisely assess the degree of screw wear without disassembling it?
1. Hidden Signs of Screw Wear
In many cases, people only realize the screw is worn out when serious quality problems appear—such as obvious shark skin, localized degradation causing black spots, or when output just won’t increase no matter how much you raise the screw speed. By then, the situation is already severe.
Screw wear essentially means the outer diameter of the flight becomes smaller, which increases the radial clearance between the flight and the barrel. Once there is leakage through this clearance, material will flow backward under pressure—that is, backflow and leakage intensify. This directly leads to two consequences:
❌ Sharp increase in shear heat: The material is repeatedly kneaded and over-sheared in the enlarged gap, causing localized temperature runaway.
❌ Difficulty building melt pressure: Especially in the metering and homogenizing zones, the material cannot be contained. In extrusion, this shows as a steep drop in output; in injection molding, it shows as longer plasticizing time and the screw continuously creeping forward during holding pressure, unable to maintain pressure.
2. Four Precise Diagnostic Methods Without Pulling the Screw
Since we’re not disassembling, we must rely on data, physical characteristics, and special minimally invasive techniques to deduce the condition.
Method 1: Melt pressure and position data testing
This is the most practical blind testing method in industry. Under the premise of normal production, same material, and unchanged process temperature, record the key data under current parameters.
✅ For extruders, monitor pressure: Keep the feed rate and screw speed constant, and compare the die pressure data with that of a new screw or normal period. If you find that to achieve the same output you have to drastically increase screw speed, and the die pressure has dropped by more than 15% compared to before, you can basically conclude that the flights in the metering section are severely worn. At the same time, the pressure fluctuation curve will change from regular small oscillations to irregular large swings.
✅ For injection molding machines, observe position and time: Check whether the plasticizing (metering) time has significantly increased. Also, during the injection and holding pressure phase, closely monitor the screw’s final position. If the screw continues to advance slowly and continuously during holding pressure, it indicates that melt is leaking backward through the worn clearance or the check ring.
Method 2: 5‑minute shutdown pressure drop/backflow method
This method does not require pulling the screw; it only needs to be performed during a material change or a brief partial shutdown:
✅ Fill the barrel with melt, preferably a standard test material with stable viscosity, such as a high‑MFI polyolefin or a special purging compound.
✅ Stop feeding, keep the screw stationary, and use the pressure sensor at the end of the die, or manually apply a back pressure to the outlet (for injection molding machines, you can manually give a slight forward movement).
✅ Observe the rate of pressure decay. If the clearance is normal and the melt seals well, the pressure will drop very slowly; if the screw is worn and the clearance is too large, the melt will leak back rapidly, and the pressure curve will show a cliff‑like drop.
Method 3: Process reverse deduction—look for abnormal temperature and torque
Pay attention to your control panel; the machine itself is actually warning you.
✅ Abnormal drop in torque and current: When material, temperature, and screw speed are completely identical, if you find that motor torque and current are significantly lower than historical normal values—say, a 10%–20% drop—don’t celebrate too soon. This isn’t the machine saving power; it’s because the clearance has increased, the material leaks through, and the viscous resistance on the screw has decreased.
✅ Heater bands stop working, cooling fans run wild: Under normal conditions, the barrel needs heater bands to supply heat. If wear is severe, the metering section will generate a lot of abnormal friction shear heat. You will notice that even though you set 200°C, the actual temperature keeps shooting up to 215°C, the heater bands don’t work at all, and the cooling fans or cooling water valves are on full blast all day. This kind of thermal runaway is mostly caused by excessive shear from screw wear.
Method 4: Advanced method—direct visual inspection with an industrial borescope
If you suspect wear based on the above data but aren’t 100% sure, try this. You don’t need to remove the entire screw; just find an access port on the machine while it’s shut down and the material has not completely hardened:
✅ Extruders: Remove a vent port or vacuum port on the barrel, or directly remove the die head.
✅ Injection molding machines: Remove the nozzle while it’s still hot.
Then use a high‑temperature resistant industrial pipe borescope to probe through these openings. Through the high‑definition lens, you can directly observe the local flights. On a severely worn screw, you can clearly see that the flight tops have become rounded and blunt, and there may even be obvious metal peeling or scoring. For injection molding machines, you can also take a look at the check ring wear through the front end. Although this method involves using some tools, it saves you the enormous task of pulling the screw.
3. Diagnostic Logic for Quick Reference
Scenario 1: Output cannot increase / Plasticizing time significantly longer
This usually means the feed or metering section is worn. Extrusion operators can compare the historical “speed‑output” curve to calculate slip efficiency; injection molding operators should be alert to material leakage and backflow during the holding pressure phase.
Scenario 2: Actual temperature in a certain zone seriously exceeds the set value
This is mostly due to worn flights in that zone, where excessive clearance leads to intense shear. Directly observe the current of the heater band in that zone; if it remains zero for a long time and the barrel is continuously overheating, it’s almost certainly confirmed.
Scenario 3: Die pressure fluctuates widely for no apparent reason / Holding pressure position cannot be maintained
This is a typical sign of wear in the homogenizing section, metering section, or check ring, leading to an inability to hold the material. Consider using a borescope to probe through the nearest vent port, vacuum port, or nozzle for direct observation.
Final Note
When output drops and temperature drifts, don’t rush to dispatch a team to pull the screw. Grab your logbook, pull up the data from three months ago, and check it against torque, pressure, time, and heating output. Data often tells you the truth earlier than the naked eye.
